cmake: Install libxml.m4 on UNIX-like platforms

Install libxml.m4 to follow GNU Autotools more closely

CMakeLists.txt

@@ -677,6 +677,10 @@
 configure_file(xml2-config.in xml2-config @ONLY)
 install(PROGRAMS ${CMAKE_CURRENT_BINARY_DIR}/xml2-config DESTINATION ${CMAKE_INSTALL_BINDIR} COMPONENT development)
 
+if(UNIX)
+	install(FILES ${CMAKE_CURRENT_SOURCE_DIR}/libxml.m4 DESTINATION ${CMAKE_INSTALL_DATADIR}/aclocal)
+endif()
+
 set(XML_INCLUDEDIR "-I${CMAKE_INSTALL_FULL_INCLUDEDIR}/libxml2")
 set(XML_LIBDIR "-L${CMAKE_INSTALL_FULL_LIBDIR}")